1. Field of the Invention
Embodiments of the present invention relate to a gaschromatograph-ion mobility spectrometer (GC-IMS) system.
2. Description of the Related Art
U.S. Pat. No. 5,811,059A and U.S. Pat. No. 6,481,263B1 each disclose a gas analyzing apparatus which comprises a gas chromatograph (GC) and a single-mode ion mobility spectrometer (IMS). The apparatus has a better separation capacity than a simple IMS.However, it is only used To detect charged particles of a single polarity, but neither be used to detect both positive and negative ions simultaneously, nor be used to detect those substances having opposite electroaffinities. DE19502674C1 discloses a method for measuring both positive and negative ions by switching of an electric field. Although both the positive and negative ions are measured, they are not simultaneously measured due to a switching time interval, so that correlative information of the positive and negative ions will be lost during the measurement. CN201917559U discloses a gas analyzing apparatus which comprises a gas chromatograph (GC) and a dual-mode ion mobility spectrometer (IMS), wherein both positive and negative ions of a mixture are detected simultaneously. However, in the gas analyzing apparatus comprising the gas chromatograph and the IMS, as those disclosed in patent documents including CN201917559U and U.S. Pat. No. 5,811,059A, a sample separated by the GC is introduced directly into an ionization region. An ionization source is a main functional component of the IMS. Ionization effects generated by different ionization sources have a very direct influence on performance of the IMS. For example, all of the most widely used 13-sources will emit high-energy primary electrons (of 67 keV for 63Ni and of 18 keV for 3H). If the structural design in which a sample is introduced directly into the ionization region is used, when the sample passes through in the vicinity of the β-source, it will be hit by high-energy β-particles directly into molecular ion fragments, or will be ionized into positively charged sample molecular ions. On one hand, the sample molecular ion fragments will cause a rise in Reaction Ion Peak (RIP), disturb a baseline or generate interference peaks, and reduce IMS resolution. A hard ionization source will generate complicated fragments and generate a spectrum which is difficult to discriminate, especially for biological macromolecules such as proteins and nucleic acids. As a result, it is difficult to extend the application field of the GC-IMS system to the detection field of organic macromolecules. On the other hand, the sample molecular ion fragments or the positively charged sample molecular ions will further react with reactive ions to generate unidentifiable ion mobility spectrum, which disorders spectral lines and seriously affects analysis of the spectral lines. If a pulsed corona discharge ionization source, as another most widely used ionization source, is used, the corona discharge belongs to soft ionization (primary electrons of 5 eV-10 eV), and thus it will not hit sample molecules into fragments. However, the sample molecules passing through in the vicinity of a corona needle will be ionized into positively charged sample molecular ions, and the positively charged sample molecular ions may react with unionized carrier gas molecules, which increases complexity of sample analysis and even disturbs peak analysis. Furthermore, the positively charged sample molecular ions maybe destroyed due to the neutralization reaction with negative reactive ions, so that the detection is evaded. In addition to the problems caused by the above design defects, conventional dual-mode IMS (CN201917559U) and single-mode IMS (U.S. Pat. No. 5,811,059A) have another design defect, that is, positive and negative ions generated in the ionization region are not separated from each other when entering a reaction region. When carrier gas is ionized by the ionization source, both positive ions (mainly, (H2O)nH+) and negative ions (mainly, O2−(H2O)2) will be simultaneously generated. Coulomb attraction forces will be generated among the positive ions and the negative ions generated in a same space of the ionization region. If no repulsion voltage is applied to the ionization region, the positive ions and the negative ions (or electrons) driven to enter the reaction region by a carrier gas flow will be neutralized due to their collision and recombination. This reaction region will become a trap where the positive ions and the negative ions are destroyed. Even if a repulsion voltage is applied to the ionization region in order to separate the ions having different charges, there is a loss caused by neutralization due to recombination of the ions [Siegel, M W, Atmospheric pressure ionization, in Plasma Chromatography, Carr, T W, Ed., Plenum Press, New York, 1984, chap. 3, pp. 95-113.]. Therefore, such structural design will result in a reaction ion loss, thereby resulting in a low baseline signal, and a decreased detection sensitivity of the IMS.